Many electronic devices and electrical systems, such as transistors, integrated circuits, power controls, switches, microprocessors, and the like, generate heat during operation. The capability of some electronic devices is limited by their ability to remove or expel internally generated heat. This heat must be removed from the device to avoid general or localized thermal degradation or failure of the device. In some devices, the heat generated is dissipated sufficiently by the enclosure, package, header, or leads. Other devices require additional apparatus, such as heat sinks, for removing and dissipating excess thermal energy.
For purposes of the present invention, a heat sink is any body of metal or like material which is placed in thermal communication with an electronic device package or other heat generating component for transferring internally generated heat from the device and for rapidly dissipating this heat to the surrounding environment by conduction, convection, and/or radiation. In order to accomplish this, heat sinks are generally made of materials having high coefficients of thermal conduction such as aluminum, copper, and alloys thereof. Heat sinks may be extruded, machined, molded, sawed, or formed of sheet metal bodies. A typical heat sink for electrical applications functions by conducting heat away from the heat generating component and dissipating the heat into the surrounding air. Accordingly, heat sinks are typically shaped to maximize surface area by incorporating fins or pins. Increased surface area maximizes heat dissipation from the heat sink to the surrounding atmosphere.
In order for the heat sink to operate efficiently, it must be secured to, or otherwise placed in good thermal communication with, the heat generating device. Various means have been used to attach heat sinks in thermal communication with heat generating device packages. A known practice is to glue or otherwise adhesively attach a heat sink directly to a predetermined surface of the heat generating device package with heat-conductive epoxy, thermally enhanced adhesives, solder, or the like. Heat sinks may also be mechanically attached to electronic device packages with resilient metal clips mounted on the heat sink or with screws, bolts, clamps, or other connective means which urge the heat sink and electronic device package into mutual contact. In addition, heat sinks may be remotely located but thermally coupled to a heat generating device via a heat spreader device, a heat pipe, or any other means of transferring heat from the source of the heat to the heat sink.
Recently, technological advances have allowed electronic components to decrease in size while increasing in power and speed. This miniaturization of electronic components with increased capability has resulted in the generation of more heat in less space with the electronic device packages having less physical structure for dissipating heat and less surface area for attaching a heat sink to dissipate the heat. A heat spreader can be used to increase the surface area for transferring heat. Such a heat spreader serves to disperse the heat generated by the electronic device throughout a larger physical structure than the device or device package, thus allowing the device to dissipate the heat through the increased surface area or providing greater surface area to attach a larger heat sink in thermal communication with the electronic device package.
Further complicating modern electronic thermal management is the growing preference for surface mounting electronic components on printed circuit boards (PCBs) or other substrates. The use of surface mount PCBs is desirable because this is a less costly and less time consuming process of fabricating and populating PCBs than the older manufacturing assembly process which required insertion of components through holes in the circuit board for subsequent soldering operations. Surface mount PCBs allow for increased use of automated manufacturing and assembly techniques. In particular, surface mountable devices are typically robotically picked and placed on the PCB and then soldered to the PCB in one automated manufacturing process. In addition to reducing assembly costs, the surface mount technology has allowed for even greater miniaturization of the electronic device packages used on the boards. These smaller packages further reduce the device's ability to dissipate its own heat thus increasing the need for separate heat sinks. The smaller packages, however, also make it increasingly difficult to attach a heat sink directly to the device package.
Several methods have been suggested to dissipate heat from these surface mount electronic device packages. One common approach is to use the ground plane, or other similar thermally conductive area of the PCB, as a rudimentary heat sink to spread and dissipate the heat. Although this may provide adequate dissipation of the heat generated, it typically requires significant board space, thereby increasing the size of the PCB or limiting the available PCB space for populating the PCB, both of which are undesirable side effects.
An alternative to using a portion of the PCB as a heat sink is to use larger device packages thereby providing greater surface area to dissipate the heat directly from the device package. In addition, such larger packages often provide improved means for mounting a separate heat sink directly to the package. Again, however, the use of such larger device packages and the use of separate heat sinks often require relatively large amounts of board space. This also runs contrary to the general desire to miniaturize electronic components. Another disadvantage is that the separate heat sink typically must be attached to the device package after the majority of the PCB has been soldered in an automated manufacturing process. Adding an additional step to the manufacturing process increases assembly time and costs.
Alternatively, a combination of the two concepts has been suggested wherein the surface mount device package is thermally connected to a smaller thermal plane, thermal pad, or thermal land on the PCB. A heat sink can then be soldered to the thermal pad in indirect thermal communication with the heat generating device. This alternative reduces the amount of PCB space used and eliminates the post assembly soldering of the heat sink to the PCB. One example of this technique is shown in U.S. Pat. No. 5,365,399 issued to K. Kent and J. Glomski on Nov. 15, 1994 entitled "Heat Sinking Apparatus for Surface Mountable Power Devices." One disadvantage to this method is the fact that the size of the heat sink must be limited so that the heat sink can be heated during manufacture to allow for reflow of the solder during the typical surface mount solder manufacturing process. By limiting the size or mass of the heat sink, the amount of heat dissipation is also limited.
In addition, soldering the heat sink to the PCB means the heat sink can not easily be removed from the PCB. Thus, the heat sink cannot be exchanged for different size heat sinks. The ability to interchange heat sinks is desirable because different size heat sinks may be utilized depending on the circumstances. For instance, a larger heat sink may be needed if the heat generating device is generating more heat than expected, or the heat sink is dissipating less heat than expected. Conversely, if the device is not generating as much heat as expected, or the heat sink is dissipating more heat than expected, a smaller heat sink may be used to further reduce the size of the electronic component. In addition, differing sized heat sinks may be required to compensate for different environments in which the PCB may be placed. For example, environments with high ambient temperatures or poor ventilation may require larger heat sinks.
Although the '399 patent contemplates the addition of heat sink extensions to create various sizes of heat sinks, it does not provide an efficient solution to the problem. The '399 patent discloses slots in the main body of the heat sink which allow for the insertion of a spring clip heat sink extension. The spring clip heat sink extension and slot disclosed, however, provide a relatively poor thermal connection to the heat sink which limits the effectiveness of the heat sink extension. In addition, spring means must be incorporated into the heat sink extension in order to attach the extension to the heat sink. This limits the shape, size, and materials which may be used as heat sink extensions.